Apparatus and method for determination and control of paper web qualities on a papermaking machine

ABSTRACT

The invention relates to an apparatus for the on-line determination of paper web moisture content and/or temperature on the dryer section of a papermaking machine. The apparatus is designed to form, in combination with a doctor means and/or a packet ventilating tube, such a compact structure wherein the apparatus operates underneath such means or in a close proximity thereto. The invention also relates to a method for control of paper web moisture content and temperature on the basis of measurements performed on the dryer section of a papermaking machine.

This application is the national phase under 35 U.S.C. § 371 of PCTInternational Application No. PCT/FI00/00957 which has an Internationalfiling date of Nov. 2, 2000, which designated the United States ofAmerica, and which claims priority to Application Ser. No. 19992375 inFinland, filed on Nov. 3, 1999.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to an apparatus for determination of paperweb qualities during the manufacture of the web and a method forutilizing the apparatus.

DESCRIPTION OF THE BACKGROUND

On a papermaking machine, the moisture content of the web generally isin the order of 55% after the press section, which means that more thanhalf of the web weight at this stage still is water. The cross-directionmoisture profile of the web as it leaves the press has been found toaffect the tensional cross-direction profile of the web at the winder.Further, it has been experimentally found that the shape of the webtension profile measured at the winder is also reflected in the webprofile downstream through the web finishing steps and even up to runsin the printing machine. Obviously, an uneven web tension profile causesproblems, one of which is evidenced as breaks of the web running in thepapermaking machine, the finishing equipment or the printing machine.

As to the general state of the art, reference is made to patentpublication FI 62571, wherein is described the adaptation of a blow tubeto operate with the doctor blade or in a close proximity thereof.Furthermore, it is known in the art to measure the qualifies of therunning web at the papermaking machine after the dryer section butbefore the winder.

Today, the end value of the web moisture profile is measured at ascanning beam located after the dryer section, whereby it is attemptedto control the profile as uniform as possible utilizing a steam boxlocated within the press section, unsymmetrical loading of the press anda wetting unit located within the dryer section or immediately after thesame. Possible deviations in the cross-direction moisture profileremaining in the web after the press section are diminished in the dryersection, because the evaporation of the few last percent units of theresidual moisture content demands a substantially high amount of energyinput, which in most papermaking machines gives an end moisture contentas low as 2%, whereby deviations from a uniform cross-direction moistureprofile remain insignificant. This processing approach involves suchunnecessary steps as overdrying on a portion of the web and subsequentrewetting thereof in order to keep the moisture profile of the finishedweb within preset limits. Obviously, such overdrying increases thespecific energy consumption and elevates the costs of papermaking. It isalso possible that the drying effect imposed on the web by the dryersection gives an inhomogeneous result due to soiled dryer cylinders oruneven ventilation, thus affecting in a substantial manner the moisturecontent profile of the web as it leaves the press section. A furtherfactor complicating the situation is that the outcome of any controlaction performed on the units of the press section is detectable with along delay only at the scanning beam which is located in front of thewinder. As modern papermaking machines are designed for ever faster webspeeds up to 2000 m/min, the dryer section must respectively be madelonger than in the prior art, whereby changes occurring on the wet endof the papermaking machine can be sensed at the scanning beam, which islocated downstream from the dryer section, with a delay longer than thatcustomary in the prior art. Due to these reasons, it is desirable tomeasure the qualities of the paper web, such as its moisture content andtemperature, already on the dryer section, most advantageouslyimmediately downstream from the press section.

A problem hampering flawless operation of measurement equipment andinstrumentation located on the cylinder dryer section is theaccumulation of paper web debris during web breaks into all possiblelocations on the beams, tubes and the like elements typically needed inthe system when these elements are located under the dryer cylinders.Additionally, the high ambient temperature and humidity of the operatingenvironment set specific demands on the measurement equipment andinstruments.

BRIEF SUMMARY OF THE INVENTION

The goal of the embodiment according to the present invention is toeliminate the above-described complications. The characteristic featuresof the apparatus and method according to the invention are disclosed inthe appended claims.

It is an object of the present invention to provide an apparatus for themeasurement of paper web qualities, particularly the web moisturecontent and temperature essentially on the dryer section of apapermaking machine. The apparatus according to the invention maycomprise a single measurement device or a plurality thereof adapted tomeasure the paper web qualities, particularly the web moisture contentand/or temperature, whereby at least one of the devices is located onthe dryer section of the papermaking machine, advantageously as close aspossible to the press section thereof.

The measurement device used in the apparatus is generally known as ascanning beam. The construction and principle of operation of thescanning beam may vary according to specific needs. An essentialrequirement herein is that the scanning beam can operate reliably andaccurately over its specified measurement span and to submit theinformation thus obtained to further processing, e.g. by a control meansadapted to utilize the information. Most advantageously, the scanningbeam has a construction enclosed in a box section beam that houses atleast one sensor 10 adapted to move in the cross-machine direction overthe web, whereby the sensor-actuating elements are located in theinterior of a protective enclosure and only the lens portion 11 of thesensor(s) is flush with the exterior surface of the enclosure.

A preferred embodiment has the apparatus placed to operate inconjunction with a blow box 7 or a doctor 6 so close to said unit thatno essential gap remains therebetween, thus achieving a constructionfree from the tendency of accumulating paper web debris or larger clumpsof paper. The closed structure of the scanning beam is attained by wayof adapting a seal belt 15 to the sensor(s) 10 so as to be movabletherewith, whereby the belt seals the slot of the scanning beam at allother points except the instantaneous location of the sensor(s) 10. Inanother embodiment, there is mounted a sufficiently great number of thesensors 10 into a linear array, thus dispensing with the need to movethe sensors in the cross machine direction in order to obtain scanningresults essentially over the entire width of the web 9.

Any mechanical device located on the dryer section is hampered by theaccumulation of dirt and condensation on the moving parts of the devicethat may cause jamming the movement of the device and put it out offunction. In the embodiment according to the invention, the scanningbeam is equipped with air feed means 12 for blowing air into theinterior of the beam in order to stabilize the temperature of thescanning beam and, additionally, to maintain the interior of thescanning beam by means of the air feed at a pressurized level in regardto the ambient pressure thus preventing dirt from entering the interiorof the box-section beam. This arrangement provides a reasonableoperating environment in the interior of the scanning beam for thesensor 10 enclosed therein, the sensor-moving parts and other componentsthat are sensitive to the attack of dirt and humidity.

Under certain conditions, the temperature of ventilating air blown intothe interior of the scanning beam is arranged to be controlled on thebasis of the information obtained from a temperature sensor located inthe interior of the scanning beam so as to keep the ambient temperaturein the interior of the beam at a constant level. For this purpose, theair used for ventilating the beam must be heated or cooled as requiredby means of a heat exchanger arranged to operate in conjunction with theventilation air equipment

In certain applications, it is advantageous to provide the scanning beamwith an elevating/lowering system, later called a lift actuator 18, bymeans of which the scanning beam can be lowered for maintenanceoperations to sufficiently large distance away from its operatingposition close to the dryer cylinders, thus positioning the beam at asuitable height from, e.g., floor level of the machine service dump.

The operation of the lift actuator 18 may also be connected to thepaper-making process so that, at a web break, the scanning beam movesfrom its normal scanning distance to a safety distance from the paperweb so as to provide during a web break occurring at the dryer cylinderjust upstream or downstream from the scanning beam an unobstructedfall-down path of the broke into the machine dump.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in more detail withreference to the appended drawings in which FIGS. 1–4 describe a fewexemplifying embodiments according to the invention; and

FIG. 5 shows a schematic view of the construction and operationprinciple of the scanning beam.

DETAILED DESCRIPTION OF THE INVENTION

Depending on the structure of the dryer section in the papermakingmachine, the arrangement of the equipment counting from top downwardcould be as is shown in FIG. 1 comprising a doctor 6, a blow tube 7, ascanning beam 8, or alternatively, as shown in FIG. 2 comprising adoctor 6, a scanning beam 8, a blow tube 7 or further alteratively, suchas shown in FIGS. 3 and 4.

Now referring to FIG. 1, therein is shown an exemplifying embodiment ofthe invention. Under a dryer cylinder is disposed a doctor 6 having ablow tube 7 mounted thereunder. Immediately beneath this subassembly,there is adapted a box-section scanning beam 8 that senses the moisturecontent and temperature of the running web 9. In FIG. 1 is also shown apossible rinsing spray 17 serving to flush away with a cleaning mediumany paper web debris remaining in front of the scanning beam after a webbreak.

In FIG. 2 is shown a practicable arrangement, wherein to the undersideof the dryer cylinder is placed a doctor 6 having a scanning beam 8located there-under. Immediately under the latter, there is placed ablow tube 7. At a web break, the blow tube injects air from nozzles 13that detaches paper web debris from the surface of the measurementequipment. Also during measurement in a normal running condition, theblow tube ejects dry air into the spaces remaining between the doctor 6,the scanning beam 8, the blow tube 7, the dryer cylinder 16 and the web9, thus removing excess humidity to assure a problem-free measurementenvironment.

In FIG. 3 is shown a practicable arrangement for tight machine layoutsthat prevent the scanning beam 8 from being located to a close proximityof the blow tube 7. In this case, plates or the like elements 14 areplaced between the blow tube 7 and the scanning beam 8 so as toconfigure the scanning beam with the blow tube into an integratedstructure.

In FIG. 4 is shown a practicable arrangement for tight machine layoutsthat prevent the scanning beam 8 from being located to a close proximityof the doctor 6. In this case,.plates or the like elements 14 are placedbetween the doctor 6 and the scanning beam 8 so as to configure thescanning beam 8 with the doctor 6 into an integrated structure.

In FIG. 5 is schematically shown the box-section structure of thescanning beam 8. All the essential parts are enclosed in the box-sectioninterior of the scanning beam. The scanning beam houses one or a greaternumber of sensors, whereby the sensor slot of the scanning beam iscovered by means of a seal belt 15 over all other points except theinstantaneous location of the sensor(s) that is/are moved in thecross-machine direction so that the above-mentioned web qualities can bemeasured over the entire cross-machine width.

The invention also relates to a method for determination of paper webqualities during the manufacture of the paper web. In the methodaccording to the invention, the determination of paper web qualities,more particularly web moisture content and temperature, is performedfrom the web profiles that are measured in the cross-machine directionand/or machine direction, specifically on the dryer section of apapermaking machine.

The most preferred embodiment of the method according to invention canbe implemented by way of performing the determination of paper webmoisture content and temperature on the dryer section of a papermakingmachine, particularly advantageously as close as possible to the presssection. The goal in measuring the paper web qualities advantageouslyimmediately downstream from the press section is that herein the effectof the dryer section is still insignificant, thus allowing anyunevenness possibly occurring in the web moisture profile to be detectedin an undisturbed manner. Based on such moisture profile information, itis possible to control the operation of the headbox, steam box and/orpress section so that the paper web attains a moisture profile of adesired shape at the entry of the web into the dryer section in order toachieve an optimal web tension profile, among other benefits. As thepresent arrangement allows the scanning beam to be located in a veryclose proximity to the press section, the response of control actionscan be made substantially faster and more accurate than what is possiblein a conventional control of the headbox, steam box and press sectionthat is based on measurement information obtained downstream from thedryer section.

By virtue of performing the moisture profile measurement immediatelydown-stream from the press section, it is also possible after themeasurement to carry out, e.g., by means of an afterwetting unit,correction of a defective moisture profile detected in the measurement,thus preventing a web of defective moisture profile from reaching theupwinder. Hence, the arrangement according to the invention facilitatesweb moisture profile control in a realtime fashion, if so desired,either on the wire section, on the press section or as well on the dryersection, or after any of these, most advantageously utilizing anafterwetting unit for the control.

To those skilled in the art it is obvious that the invention is notlimited by the embodiments described above, but rather, can be variedwithin the scope and spirit of the appended claims. Accordingly, theinvention may also be applied to other machines such as cardboardmachines operating in a similar manner as a papermaking machine.

1. Apparatus for on-machine determination of paper web qualities duringthe manufacture of said paper web, comprising: a dryer section of apapermaking machine; a scanning beam measurement device located in saiddryer section for determining profiles of paper web qualities measuredin a given direction; and said scanning beam measurement device beinglocated in substantially close proximity to a doctor element or blowtube so as to form an integrated structure therewith; said scanning beammeasurement device providing a scanning beam for scanning said paperweb, including at least one sensor, whereby the sensor and its actuatingelements are located in the interior of a box-section beam enclosure soas to leave only a lens portion of the sensor flush with the exteriorsurface of the enclosure, said box-section beam enclosure having asealed structure and being provided with ventilation air blowingequipment serving to keep the interior space of the beam at a controlledtemperature and pressurized atmosphere in regard to the ambientenvironment.
 2. Apparatus according to claim 1, wherein said apparatusincludes at least one scanning beam (8) measurement device suitable fordetermination of paper web qualities, whereby at least one of said atleast one scanning beam measurement device is located on the dryersection of the papermaking machine, close to the press section thereof.3. Apparatus according to claim 1, wherein said apparatus is locatedunderneath a doctor element holder beam or in a close proximity theretoor, alternatively, underneath an air-distribution means connected tosaid doctor element holder beam or in a close proximity thereto. 4.Apparatus according to claim 1, further comprising a scanning beam (8)located in a substantially close proximity to a doctor element or a blowtube so as to form an integrated structure therewith, either directly orvia plate elements (14).
 5. Apparatus according to claim 1, wherein aflow of blown air passes between a lens of the measurement device andthe web being scanned, so as to sweep through the area scanned by a beamof light in order to keep moisture and dust away from said lens and saidbeam of light.
 6. Apparatus according to claim 1, further comprising alift/transfer means for setting the scanning beam to measuring, safetyand servicing distances from the web being scanned.
 7. Apparatusaccording to claim 1, further comprising a rinsing spray tube above ascanning beam extending over the cross-machine length of said scanningbeam and particularly adapted to operate on the side having the beam oflight thereon.
 8. Apparatus according to claim 1, wherein the directionis a cross-machine direction.
 9. Apparatus according to claim 1, whereinthe direction is a machine direction.
 10. Method for on-machinedetermination of paper web qualities during the manufacture of saidpaper web, comprising the steps of: determining profiles of paper webqualities in a measurement direction of the web, which is on the dryersection of the papermaking machine; locating a scanning beam measurementdevice for determining said profiles in a substantially close proximityto a doctor element or blow tube so as to form an integrated structuretherewith; said scanning beam measurement device providing a scanningbeam for scanning said paper web, including at least one sensor, wherebythe sensor and its actuating elements are located in the interior of abox-section beam enclosure so as to leave only a lens portion of thesensor flush with the exterior surface of the enclosure, saidbox-section beam enclosure having a sealed structure and being providedwith ventilation air blowing equipment serving to keep the interiorspace of the beam at a controlled temperature and pressurized atmospherein regard to the ambient environment.
 11. Method according to claim 10,wherein the scanning beam (8) is located on the dryer section of thepapermaking machine, close to the press section thereof.
 12. Methodaccording to claim 10, wherein the paper web qualities measurementinformation is utilized in a realtime fashion during the manufacture ofthe paper web.
 13. Method according to claim 10, wherein a headbox, wiresection, press section, dryer section, a steambox and/or an afterwettingunit is/are controlled on the basis of the thus obtained measurementinformation, related to the moisture content and/or temperature of theweb, so as to achieve a desired end result.
 14. Method according toclaim 10, wherein the direction is a cross-machine direction.
 15. Methodaccording to claim 10, wherein the direction is a machine direction.